It's important to build a straight airplane otherwise it just won't fly as well as it could. Besides, I'm anal as heck when it comes to this sort of thing so I just can't have enough straight edges. I went out and bought a bunch of aluminum angle. I then cut it up into various lengths. I then attached some self sticking mangetic tape to many of them.

I have straight edges ranging from 6' long to 2" long. The really short ones are used more for ensuring a part is perpendicular to the table than it is for ensuring straightness.